Mass analyzers for detecting and analyzing trace substances require that ions of the substance to be analyzed be introduced into a vacuum chamber containing the mass analyzer. It is often desired to perform elemental analysis, i.e. to detect and measure the relative quantities of individual elements in the trace substance of interest. U.S. Pat. No. 4,501,965 assigned to MDS Health Group Limited, the assignee of the present invention, describes a method and apparatus for conducting elemental analysis, in which the trace substance of interest is introduced into a high temperature plasma to reduce it to its individual elements. The plasma produces predominantly singly charged ions of the elements, which are directed through a small orifice into the vacuum chamber and which are then focussed into the mass analyzer.
Although instruments such as that described in the above mentioned U.S. patent operate well under laboratory conditions, they have been found to drift under some conditions of every day use. In other words, the detected ion signal may vary substantially over a period of time even when the concentration of the element being detected in the input sample remains constant. Even worse, the drifting is found to be markedly different from one element to another. For example, with constant input concentrations of elements A and B, the ion signals detected might decrease considerably over a period of time for element A and increase for element B. The drifting was found in some cases to be so large, rapid and non-uniform that recalibration of the machine at very frequent intervals was required, which was a severe nuisance.